1. Field of the Invention
The present invention relates generally to a braking system for a motor vehicle, and more particularly to improvements in an anti-rock-back device incorporated in the braking system, for preventing or minimizing a rock-back motion of the vehicle driver/passengers which occur when the vehicle is stopped with brake applied.
2. Discussion of the Prior Art
A motor vehicle is brought to a stop by keeping a brake pedal depressed. When the vehicle is stopped, the vehicle driver and passengers are subject to an uncomfortable backward motion (hereinafter referred to as "rock-back motion"). When the vehicle is stopped, the running speed of the vehicle (hereinafter referred to as "vehicle speed") is lowered and eventually zeroed. The rock-back motion takes place due to sudden zeroing of the deceleration rate (negative acceleration rate) of the vehicle from a given value. To assure a comfortable stop of the vehicle without the rock-back motion, the vehicle driver is required to exercise an utmost in manipulating the brake pedal, particularly at a terminal portion of the braking operation immediately before the vehicle is brought to a complete stop. This manipulation of the brake pedal is difficult for an unskilled vehicle driver, and is not easy even for a skilled driver.
There is known a braking system which has a function of gradually or slowing lowering the vehicle deceleration rate to zero, during application of a braking pressure to the wheel cylinders of the vehicle, so that the amount of the rock-back motion of the driver and passengers upon stopping of the vehicle is minimized.
An example of such a braking system having an anti-rock-back arrangement is disclosed in laid-open publication No. 56-129341 of Japanese unexamined Utility Model Application. In this braking system, a rod for transmitting an operating force of a brake pedal to a master cylinder piston is connected to the brake pedal by means of a link mechanism which is provided with a solenoid. When the brake pedal is depressed, the rod is advanced with the brake pedal, thereby producing a braking pressure. Immediately before the vehicle speed is zeroed, the solenoid is energized to activate the link mechanism, so as to permit a movement of the rod relative to the brake pedal. Thus, the rod may be retracted while the brake pedal remains at the operated position, and the brake is automatically released.
Since the brake is released immediately before the vehicle speed is zeroed, the amount of the rock-back motion upon stopping of the vehicle may be reduced, as if the driver intentionally reduces an operating force on the brake pedal at a terminal portion of the brake application.
The fact that the link mechanism permits the rod to be retracted means that the link mechanism also permits the brake pedal to be advanced in the operating direction. Accordingly, the brake pedal is suddenly depressed a further distance upon stopping of the vehicle, which causes the vehicle driver's foot to be unexpectedly moved. This is also uncomfortable to the driver.
To sufficiently reduce the amount of the rock-back motion of the vehicle driver and passenger, it is desirable that the wheel cylinder pressure be smoothly or slowly lowered. However, the above proposed arrangement suffers from a sudden drop of the wheel cylinder pressure, and is not sufficiently capable of preventing the rock-back motion.
In view of the above drawbacks in minimizing the rock-back motion upon stopping of the vehicle by brake application, it is considered to lower the braking pressure in the wheel cylinder in a controlled fashion, a suitable time before the vehicle is completely stopped.
If the anti-rock-back brake control operation by lowering the wheel cylinder pressure is performed uniformly in a predetermined manner in different conditions of the road on which the vehicle is running, the vehicle may not be stopped with a suitable braking distance, or the rock-back motion of the driver and passengers may not be sufficiently reduced. More specifically, if the anti-lock-back lowering of the wheel cylinder pressure is effected on an uphill or downhill road, in the same manner as practiced on a level road, the braking distance may be unnecessarily increased, or the amount of pressure decrease may not be sufficient, with a result of an insufficient anti-rock-back effect.
Further, if the anti-rock-back brake control operation is effected uniformly irrespective of the deceleration rate of the vehicle prior to starting the anti-rock-back brake control operation, the required braking distance or the anti-rock-back effect may be fluctuated. In this respect, it is noted that the deceleration rate of the vehicle at a given point of time during the brake application is almost proportional with the wheel cylinder pressure at that point of time, provided the road surface condition is constant. Accordingly, the braking distance and the anti-rock-back effect obtained may vary depending upon the wheel cylinder pressure at which the anti-rock-back brake control operation is started.
It is considered that the anti-rock-back brake control operation may be achieved by utilizing an anti-lock braking arrangement of the braking system, which includes a solenoid-operated pressure control valve device adapted to achieve an anti-lock operation to prevent an excessive amount of slip of the wheels of the vehicle during brake application by a hydraulically operated brake device. The pressure control valve device is operable between a pressure-increase position for feeding a brake fluid into the wheel cylinders to increase the braking pressures, and pressure-decrease position for discharging the brake fluid from the wheel cylinders to decrease the braking pressures. If this pressure control valve device is switched to the pressure-decrease position when the vehicle speed is lowered down to a predetermined reference value during normal brake application, the wheel cylinder pressures may be lowered so as to reduce an amount of a rock-back motion of the vehicle driver and passengers upon stopping of the vehicle. During the anti-lock braking operation, the solenoid-operated pressure control valve is required to permit a considerably high rate of a discharge flow of the fluid from the wheel cylinder in order to prevent or reduce an amount of slip of the appropriate wheel. During the anti-rock-back braking operation, on the other hand, the pressure control valve is required to permit a relatively restricted flow of the fluid from the wheel cylinder, so that the wheel cylinder pressure is slowly lowered to prevent or minimize the rock-back motion of the driver and passengers. Therefore, the same pressure control valve device cannot serve satisfactorily for both of the anti-lock and anti-rock-back brake control operations.
It is considered that the solenoid-operated pressure control valve further has a pressure-hold position for maintaining the wheel cylinder pressure. In this case, the anti-rock-back brake control operation may be achieved by switching the pressure control valve alternately between the pressure-decrease and pressure-hold positions, so that the wheel cylinder pressure may be slowly lowered. In this case, however, the wheel cylinder suffers from pressure pulsation due to the alternate switching of the pressure control valve, which causes vibrations and noise of the vehicle.
The above drawbacks are experienced where the anti-rock-back brake control operation is performed by utilizing a solenoid-operated pressure control valve device which is used for effecting a traction control operation to prevent an excessive amount of slip of the drive wheels during acceleration of the vehicle, or for effecting a braking effect control operation to provide a braking effect which is predetermined depending upon an operation of the brake pedal.
An anti-rock-back brake control operation as discussed above is generally desirable for the reason also discussed above, also, however, when a brake is applied for any emergency purpose, for example, to avoid a collision of the vehicle with a pedestrian or another vehicle, or to prevent a wheel or wheels of the vehicle from moving out of a road. In this case, the vehicle should be stopped in a time period as short as possible. Therefore, the anti-rock-back brake control operation should be avoided during the emergency brake application, in order to stop the vehicle with a short braking distance.